Mr. Dagan speaks in Tel Aviv’s Rabin Square on March 7, 2015. (Jack Guez/AFP/Getty Images)

Meir Dagan, the long-serving chief of Israel’s Mossad spy agency who was widely admired in his country for directing assassinations and acts of sabotage that helped defuse threats that included the Iranian nuclear program, died March 17 in Tel Aviv. He was 71.

Mr. Dagan, who had received a liver transplant in Belarus in 2012, less than two years after stepping down from the Mossad, died of cancer, according to reports by news outlets, including the Times of Israel.

Israeli Prime Minister Benjamin Netanyahu — recently the target of fierce criticism by Mr. Dagan over Israeli policy toward Iran — lauded the former spymaster in a statement Thursday as a “daring fighter and commander who greatly contributed to the security of the state in Israel’s wars.”

Even among the commandos of the Mossad and the Israel Defense Forces, where he rose to the rank of major general, Mr. Dagan stood out for the fierceness of his dedication. Prime Minister Ariel Sharon appointed Mr. Dagan to his Mossad post in 2002, reportedly charging him with giving the espionage agency “a knife between its teeth.”

Less than a decade later, according to Israeli news accounts cited by the New Yorker magazine, Netanyahu observed that Mr. Dagan’s impact on the Mossad had been even greater than Sharon expected. “Some people have a knife between their teeth,” Netanyahu remarked when Mr. Dagan stepped down. “Meir has a rocket-propelled grenade between his teeth.”

In his operations, Mr. Dagan was reported to have employed techniques including car bombing and poisoning. He was regarded as one of the most effective leaders in the history of the Mossad, whose earlier exploits had included the capture in Argentina in 1960 of Adolf Eichmann, the German SS officer who was a principal architect of the Holocaust. Eichmann was later tried and executed in Israel.

The works of the Mossad are shrouded in secrecy, but news reports linked the agency, under Mr. Dagan’s leadership, to operations that included the car bombing in Damascus, Syria, in 2008 that killed Imad Mughniyah, a leader of Hezbollah, a Lebanon-based proxy of Iran. The CIA reportedly assisted with the targeting of Mughniyah, who was tied to attacks on the U.S. Embassy in Beirut and the Israeli Embassy in Argentina.

Mr. Dagan was said to have directed the agency’s activities mainly toward Iran, a major Israeli enemy in the Middle East. During his leadership, the Iranian nuclear program encountered roadblocks that included laboratory fires, equipment malfunctions, plane crashes and Stuxnet, a destructive computer virus. ­Iranian scientists were killed.

“Without this man the Iranian nuclear program would have taken off years ago,” a correspondent, Ashraf Abu al-Haul, wrote in the Egyptian daily Al-Ahram. The newspaper described Mr. Dagan as “Superman.”

At times during Mr. Dagan’s leadership, perhaps emboldened by its successes, the Mossad appeared excessively brazen. The agency was linked to the assassination in 2010 of Mahmoud al-Mabhouh, a leader of the Palestinian militant group Hamas, at a hotel in Dubai. The alleged assassins carried false foreign passports — a detail that created a diplomatic stir — and were caught on camera disguised in tennis gear that appeared to lack the usual Mossad sophistication.

Mr. Dagan enjoyed considerable clout in Israel. Netanyahu called on him at Mr. Dagan’s office, rather than the traditional opposite arrangement, Newsweek magazine reported.

After leaving office, Mr. Dagan became a surprisingly outspoken critic of the prime minister for dangling the possibility of an Israeli military strike on Iran. He drew worldwide notice in 2011 when he said publicly that such a plan would be a “stupid idea” that could precipitate a protracted conflict. He instead supported covert action in Iran. In 2015, he criticized Netanyahu’s opposition to the Iranian nuclear deal.

“Israel is a country surrounded by enemies, but the enemies do not scare me,” Mr. Dagan said at a rally in Tel Aviv’s Rabin Square in March 2015, shortly before the elections in which Netanyahu was elected to a fourth term. “I am scared of our leadership.”

Mr. Dagan was born Meir Hubermann in 1945 in Kherson, Ukraine, according to his official Mossad biography. Several news profiles reported that he was born on a train en route from Siberia to Poland.

Mr. Dagan traced his zeal for Israel to the slaughter of European Jewry during the Holocaust. He kept in his office a photograph of a Jewish man, draped in a prayer shawl, kneeling with his hands raised before two Nazis. The man was his grandfather, about to be killed.

As a boy, Mr. Dagan immigrated to Israel with his parents, who opened a laundry near Tel Aviv. After his schooling, he joined the military and soon became known for his mettle. One comrade told Newsweek that, during free time, Mr. Dagan would throw knives at trees and telephone poles. He was wounded twice in battle, including during the Six-Day War of 1967.

In the 1970s, Sharon, then general of the southern command, selected Mr. Dagan to lead a special forces unit in Gaza. Mr. Dagan was credited with creating a group of military personnel who disguised themselves as Palestinians to apprehend or kill enemies. Once, his men reportedly rode a fishing boat into Gaza and executed a group of Palestine Liberation Organization operatives.

After his military career, Mr. Dagan served in Israel’s counterterrorism bureau, eventually leading the organization, and on the Israel Defense Forces’ General Staff before joining the Mossad.

As hobbies, Mr. Dagan collected swords and painted.

Survivors reportedly included his wife and three children.

When Mr. Dagan retired from the Mossad, the Jerusalem Post reported that “none of his predecessors can match, nor is it likely that any of his successors will equal, the daring and sheer volume of the covert operations he launched against his chosen targets.”

By their nature, many of those operations will remain unknown.

“Israelis,” former prime minister and defense minister Ehud Barak once said, “owe Meir Dagan a great debt of gratitude even though we cannot tell them all the reasons why.”